Observations of radio-induced optical emissions at 557.7 nm and 630.0 nm during the EISCAT-Heating experiment with pump frequencies above and near the fourth gyro-harmonic
Swedish Institute of Space Physics
This report presents results of analysis of multi-station observations of heating-induced optical emissions during the EISCAT-Heating experiment on 16 February 2015. In this experiment the heating facility operated from 16:00 UT to 17:20 UT in the O-mode with heating cycle 150 s on/ 85 s off. The heating beam was directed at magnetic zenith. The eight first heating cycles were conducted with a pump frequency of 6.2 MHz while during the five last cycles the heating frequency was 5.42 MHz. Optical observations were made by three ALIS stations (Abisko, Kiruna and Tjautjas). We found that the behaviour of the optical emissions differs significantly for the two different pump frequencies. The 630.0 nm emission intensity decreased sharply from 260 R to 100 R when the frequency of the HF heating radio wave was changed. At the same time, the 557.7 nm emission intensity did not change and was ~ 60 R. The 630.0 nm intensity reduction may be explained by the fact that for pumping with a frequency of 6.2 MHz, the main source of this emission is ionospheric thermal electrons with temperature strongly enhanced by the HF heating. However the lower heating frequency of 5.42 MHz is very close to the fourth harmonic of the local gyro-frequency. For these conditions the upper-hybrid turbulence that is probably responsible for electron temperature enhancement is suppressed by the gyro-resonance and the ionospheric electrons do not contribute to the 630.0 nm emission excitation. The electron temperature measurements by the EISCAT UHF radar support this hypothesis. Another feature found in the emission behaviour for the two different frequencies is various growth time for the 557.7 nm emission. For the high frequency pumping the 557.7 nm intensity reached steady-state value in less than 15 s. On the other hand, for the lower pump frequency the characteristic growth time of the 557.7 nm emission exceeds 30 s. The report also discusses differences in the spacial distribution of the emissions for the different heating frequencies.
Created 2015-11-06 10:25:49 by Mats HolmstrÃ¶m Last changed 2016-06-15 09:30:07 by Mats HolmstrÃ¶m